This invention relates to optical systems, and, more particularly, to an optical system having static and dynamic aberration correction.
An optical sensor receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. Optical sensors are available in a variety of types and for wavelengths ranging from the ultraviolet, through the visible, and into the infrared. Optical sensors are used in a variety of commercial and military applications. In some applications the optical sensors are fixed in orientation, and in others the optical sensor is movable such as by a pivoting motion to allow sensing over a wide angular range.
The optical sensors generally employ a photosensitive material that faces the scene and produces an electrical output responsive to the incident energy. The photosensitive material and the remainder of the sensor structure are rather fragile, and are easily damaged by dirt, erosion, chemicals, or high air velocity. The sensor is therefore placed behind a window through which it views the scene and which protects the sensor from such external agents. The window must be transparent to the radiation of the operating wavelength of the sensor and resist attack from the external forces. The window must also permit the sensor to view the scene over the specified field of regard, which is the angular extent over which the sensor must be able to view the scene. The field of regard may extend over wide angles and in two rotational directions. For example, a movable look-down sensor on a high-speed aircraft must have a field of regard that extends over specified pointing angles from front-to-back (elevation angle) and from side-to-side (azimuthal angle).
Desirably, the window introduces minimal wavefront distortion of the scene over the field of regard of the sensor, particularly if the sensor is an imaging sensor. The larger and thicker the window, however, the more likely is the introduction of wavefront distortion. It is not currently possible to design a window that is both aerodynamically acceptable and also avoids the introduction of wavefront distortion by the window in practical applications. For conformal windows that have relatively good aerodynamic properties, the amount and type of wavefront distortion vary widely as a function of the pointing angle of the sensor relative to the window. For example, the wavefront distortion of the image when the sensor is pointed straight down is quite different from that when the sensor is pointed at a large azimuthal angle to the side.
Where there is wavefront distortion introduced by the window, as is always the case to some degree, it is desirable that such wavefront distortion be correctable.
One approach to correction of aberration is through electrical manipulation of the output signal of the sensor in an attempt to at least partially negate the distortion produced by the window. While operable to some extent, electronic aberration correction has a limited dynamic range due to the limitations of digital electronic signal processing. Additionally, the application of complex correction algorithms slows the processing of the image.
Another approach is to provide an optical corrector that alters the image before it reaches the sensor so as to at least partially negate the effects of the wavefront distortion caused by the window through the introduction of a counter-distortion in the optical beam. For use with conformal windows, such a corrector must be dynamic in nature so as to be adjustable according to the pointing angle of the sensor. It must have a wide dynamic range to accommodate the wide range of distortions encountered as a function of the pointing angle of the sensor.
Dynamic optical correctors such as deformable mirrors and dynamic coma and astigmatism correctors are known. See, for example, U.S. Pat. No. 5,526,181. The available dynamic optical correctors are operable for some applications. However, deformable mirrors are very complex and costly, and have a limited range of operation. The dynamic aberration corrector of the '181 patent is an important advance in the art and is useful for some window configurations. Since that development, however, conformal windows with better aerodynamic properties, and associated greater requirements for aberration correction, are under development. The dynamic aberration correction of the '181 patent is insufficient to meet the performance demands of these advanced conformal windows, and a better approach is now required.
There is a need for an improved dynamic optical corrector. The present invention fulfills this need, and further provides related advantages.